1. Field of the Invention
The present invention relates to golf clubs. More particularly, the present invention relates to a golf club having a club head sole modified to expand the sweet area downward of the face surface and to increase the traveling distance of golf balls.
2. Description of the Related Art
There are various golf clubs prepared for varying conditions of golf courses. For the middle- or long-range first shot, golf clubs called “drivers” are usually used in order to extend the ball traveling distance. Because the traveling distance affects the score directly, the position of the ball impact point on the golf club head is an important factor. The hitting surface of the golf club is called “face”. Under any conditions, the user usually strikes the ball on the face of the club head.
At address, the center of gravity of a driver club head as projected on the club face is located above the center of the face as seen from a direction perpendicular to the face surface. The reason for this is as follows. The club head has an approximately inverted trapezoidal or triangular configuration in terms of the face configuration, in which the upper side is wide and the lower side is narrow. Accordingly, the mass of the golf head is inevitably deviated toward the upper side. Furthermore, because a part known as “hosel”, into which a shaft is to be inserted, is provided on the top of the club head, an extra mass is added to the upper side of the head.
Regarding the center of gravity, even a club head in which the center of gravity is located at a position approximately 60% from the bottom surface of the sole, in terms of the face height, for example, is called a “low-center of gravity model”. A ball striking area called “sweet area” is in the vicinity of the center of gravity and is an area capable of sending the ball the farthest distance. Therefore, in order to make use of the maximum repulsion capacity of the head and to obtain a long traveling distance, it is usually necessary to strike the ball on a sweet area above the center of the face. However, not only amateur golfers whose ball impact point is likely to vary, but also professional golfers occasionally shift their ball impact point intentionally according to golf course conditions.
For example, in a head (against) wind, golfers usually hit the ball in such a manner as to produce a low ball trajectory because if it is hit in the usual manner, the ball will have to fly against the wind, resulting in a failure to get the desired ball traveling distance. In this case, the ball impact point is in a lower area of the club face. This, however, results in a decrease of the repulsive force and the ball traveling distance becomes less than that obtained by striking on the sweet area. This can be explained as follows. Because the center of gravity is located at the upper side of the face, as mentioned above, that is, because the sweet areas, which is a high-repulsion area, is in the vicinity of the center of gravity, if the impact point is off this area, repulsive force is reduced undesirably.
Under these circumstances, various methods have been suggested to enable a repulsive force comparable to that obtained at the conventional sweet area even at a lower point of the face, thereby resolving the aforesaid problems. For example, Japanese Patent Application Laid-open No. 2002-17912 disclosed a golf club in which an area of the club face is specified and a coefficient of restitution is set to minimize the decrease in ball traveling distance even at the time of offset impact. Further, U.S. Pat. No. 6,524,198 described a method wherein a weight portion is provided in the lower position of the club head or a sweet area is enlarged in the downward direction, as a method of lowering the center of gravity and consequently increasing the repulsive force at the lower point of the club face.
On the other hand, Japanese Patent Application Laid-open No. 2000-176056 disclosed a technique by which a reinforcing rib is provided on the club face and the traveling distance of the golf ball is increased, without providing extra deformation to the sole or crown at the time of impact, as a method of increasing the rigidity of the golf club. Further, Japanese Patent Application Laid-open No. 2001-54596 disclosed a method for forming a plurality of metal shells on the inner side of the head and a portion adjusting the ball sound by build-up welding on the inner wall of the side peripheral portion, thereby suppressing strains.
As has been stated above, various methods have been devised to increase the ball traveling distance under various conditions. However, the proposed methods have not yet satisfactorily solved the problems. There is still room for improvement, particularly in terms of enhancement of repulsion at a lower point off the sweet area. The above-described method where the specific area of the club face is restricted and the coefficient of restitution of the specific zone thereof is set is implemented by reducing the thickness of a specific region of the club face so that the thickness varies from the center to the peripheral edge of the face, thereby consequently enhancing the repulsion effect.
However, though the effect is obtained in specific locations, the repulsion effect cannot be reliably increased and maintained in the lower positions of the face. Decreasing the head thickness is by itself limited because it decreases rigidity and reduces thickness. If the thickness is too small, it may conversely degrade the repulsion. Furthermore, a method wherein a weight portion is provided is effective in its own way but limited under the recent tendency for club head to increase in volume.
Thus, when a club head becomes large in size, the addition of a weight portion gives rise to a new problem that the mass of the head becomes large. Further, all the aforementioned techniques of increasing the rigidity of the golf club are merely the techniques of increasing the rigidity of the entire body, and none of them has a double object of increasing the coefficient of restitution by decreasing rigidity of one part and increasing the traveling distance by increasing rigidity in the other part of the same golf club.
Further, the golf industry is a world where tradition is valued originally. Substantial changes in the configuration, weight, etc., of the club head from those of the conventional one require users to change their golf swing and so forth. This may cause the swing rhythm to be destroyed. Even if an epoch-making golf club is developed, it will take a long time for the new golf club to become established in actual practice. Therefore, in terms of golf club appearance, it is ideal to develop a golf club improved in function to satisfy golfers, without substantial change in the configuration of the presently established golf clubs.
Accordingly, it is desired to develop a golf club capable of enlarging the repulsion area, particularly, downward on the club head face, without placing specific limitations on the function of the golf club, whereby the repulsion performance is improved over the conventional one even at a lower point of the club head face as well as in the conventional sweet area, and thus the ball traveling distance can be increased with high stability even under headwind conditions.